The utility of thermoplastic polyesters in engineering type applications is limited where toughness and high impact strength are required. The impact strength of unmodified thermoplastic polyesters is too low for many applications especially at a temperature below the glass transition temperature (hereinafter it is Tg) of the polyester used.
The improvement of the toughness and impact strength of thermoplastic polyester has been the subject of considerable research and development by those who are highly skilled in the art. Much of such earlier research and development has been directed to the admixture of additives with the polyester, with particular attention being given to the addition of rubber-like or elastomeric materials, such as ethylene-propylene copolymer (EPM) or ethylene propyleneterpolymer (EPDM) to improve impact strength and toughness without interfering with the other desirable properties of the polyester. However, the desired level of improvement has not been achieved with the addition of such rubber-like or elastomeric materials because of the relative incompatibility between such rubber-like or elastomeric materials and polyester resins.
Therefore, in order to overcome these problems there has been a number of studies made for increasing the compatibility between polyesters and rubber-like materials, such as by the grafting of functional groups thereon or by increasing the molecular weight of the polyester whose groups are modified by a chemical reaction such as branching and crosslinking.
In order to improve the impact resistance, methods modifying with only elastomers such as EPR(ethylene-propylene copolymer) or PIB(polyisobutylene) are disclosed in JP Patent Publication No. 71-5225; DE Patent Nos. 2,348,337; 2,650,870; and 2,901,576; U.S. Pat. Nos. 4,393,153; 4,180,494; 4,096,202; and 4,034,013. Also, methods for modifying elastomers by introducing functional groups to increase the compatibility between polyester and elastomers are disclosed in JP Patent laid-open No. 85-28446, European Patent No. 33393 and PCT International Publication No. 8604076.
As a method for modifying the polyester as a matrix resin, European Patent No. 180471 discloses a method improving impact resistance by adding an epoxy resin of DGEBA (diglycidyl ether bisphenol A) type which is reacted at the end groups of polybutylene terephthalate (PBT). In cases in which a DGEBA-type epoxy is used, the branching or crosslinking effect of the epoxy is inferior to that of the multifunctional epoxy resin having three or more oxirane groups. This would result in the blow moldability capability, which is a criteria for judging the relative ease of moldability. The blow moldability is defined as the ratio of the intrinsic viscosities of a resin at 0.1 rad/sec of shearing velocity and at 100 rad/sec of shearing velocity, that is R*=.eta.(R=0.1)/.eta.(r-100). As shown in appending figure, using bisphenol type difunctional epoxy (2) increases only the viscosity, according to increasing molecular weight, while remarkably, the increasing of R* as well as the viscosity, results from the use of multifunctional epoxy (1), and thus good blow moldability is also obtained.
The reaction mechanism on said reaction effect is as follows. Reaction mechanism (I) represents reacting difunctional epoxy resin having bisphenol groups (Epirez 510) with PBT resin. The following reaction mechanism (II) represents reacting multifunctional epoxy resin (Epiclon N665) with PBT resin.